1. Field of the Invention
Applicant""s invention relates to a purified natural zeolite pigment composition for filling and/or coating paper. More particularly, the present invention relates to a purified natural zeolite pigment composition that can be used for coating paper that produces a paper that exhibits improved characteristics over existing uncoated and coated papers made with other pigments.
2. Background Information
Pigments are used in papermaking and paper coating to improve the appearance, optical properties and printability of papers. Commonly used pigments include kaolin clay, calcium carbonate, titanium dioxide, alumina trihydrate and polystyrene. These pigments are useful in manufacture of conventional printing and writing papers and paperboards that are printed or imaged by common processes including offset lithography, gravure and xerography. Recently developed imaging technology has created needs for new types of coated and uncoated papers with properties not achievable with conventional pigments. Ink jet printing is a useful example.
Ink jet printing technology has undergone several changes in addressing the demands of existing and future digital printing applications that require high quality printed images. High quality ink jet printing typically occurs on coated paper; therefore, to produce such high quality printed images the coating composition and the ink formulation must be considered.
Current ink jet papers rely on the novel properties of the coating material to create desired properties to dry and set the ink solutions. Jet inks typically contain 2.5% by weight of organic dyes. The dye is fixed to the paper surface either by evaporation of a base such as ammonia, by migration of a base such as diethanolamine into the paper, or by changes in ionic environment when the ink meets the coating material layer.
The paper must exhibit unique properties in order to produce a high quality printed image when the ink is fixed to the paper surface. Once the ink drop is accepted by the paper, the ink must adhere to the paper and spread minimally in all directions to generate sharp edges for print contrast1 and image fidelity. The paper must be smooth to give high print densities2. In addition, the paper should minimize bleeding3 and wicking while promoting the absorption of ink to set the dye onto the coated surface since this promotes higher print densities. Ink jet droplets must be adsorbed quickly to avoid image smearing and multiple drop splatter. The dyes should be deposited near the paper surface to maximize color density and contrast while minimizing show through4.
1 Contrast is defined as the tonal change in color from light to dark. 
2 Density is defined as the degree of color or darkness of an image. 
3 Bleeding is defined as ink traveling into the sheet. 
4 Show through is defined as printing that is visible from the backside of a sheet, or the next sheet, under normal lighting conditions. 
Coating, which generally contains pigment, binders, and additives, is applied to the paper surface to improve the properties of the paper. The ink interacts with the coating to produce a high quality image. The coating prevents the ink from penetrating into the substrate. More specifically, the coating can optimize drying time for high water content dyes and separate the water-soluble organic dyes from the water vehicle and hold the dye on the surface so it doesn""t strike through to the base sheet. Smoothness and thickness of the coating layer are two important physical properties that impact print quality. Pore structure and contact angle wettability effect print quality by preventing ink spreading. In order to prevent wicking and feathering5, it is important that the thickness of the coating layer be homogenous to a scale of a few microns in depth which also helps in the absorption of successive droplets of ink at high delivery rates and any water present.
5 Feathering is defined as the spreading of ink at the edges of printed type, caused by irregularities in the ink or its distribution. 
Paper made for ink jet printing should have a hydrophilic, high porosity surface with no macroscopic structure in order to absorb ink jet droplets quickly with little spreading, wicking or dye penetration. Therefore the preferred coating for the paper surface should contain a highly porous, high surface area pigment that wets almost instantly with water. If the coating has sufficient thickness and void volume, it should be able to absorb successive droplets in multicolor printing at the highest delivery rates of commercial ink jet printing. The dye should react with the coating material to make it waterfast and rub resistant. The coating should have near neutral or alkaline pH to avoid shifts from the intended color of the dyes.
The rate of ink penetration has a large effect on final optical density through its effect on drying time and setting of the dye on the coated surface. The rate of ink penetration can be explained by the Lucas Washburn Equation of capillary flow:
I2=yr(cosxcex8)t/4v
where I is the depth of ink penetration, r is the pore radius, t is time, y is the surface tension, xcex8 is the contact angle, and v is the viscosity of the ink. In generating high print quality, the rate of ink penetration must be modified to allow sufficient wetting to occur. The hydrophilic/hydrophobic surface chemistry of the coating plays an important role in the development of image quality through the control of dot gain. Sufficient dot gain requires the dot spreading on a smooth surface and is a function of contact angle. The contact angle is itself a function of the interactions between the surface tension of the liquid, surface vapor, and liquid vapor interfaces. The determination of sufficient dot gain can be characterized through the surface tension of the interfaces from Young""s equation:
Yslv=Ysi+Yivcosxcex8
This equation evaluates the development of the contact angle which controls spread of liquid through the surface tensions involved. If the contact angle is less than 90 degrees, surface roughness will reduce the contact angle even more. Whereas if the contact angle is greater than 90 degrees the surface roughness will increase the contact angle. Porosity also effects the measured contact angle.
The interactions between ink and the coated substrate play a vital role in producing images that are long lasting, well defined and of high strength regardless of printer application. The main interaction occurs at the surface of the substrate, where the type of bonding that occurs between the colorant and the media dictates the final print quality. The interactions that take place between the colorant and the plain paper are controlled by hydrogen bonding and Van der Waals forces, while ionic and electrostatic forces are responsible for the interactions between the colorant and the coated paper.
Hydrogen bonding is the most significant bonding that takes place between color and media, where cellulosic material is involved. For a large dye molecule, a large number of sites are available for hydrogen bonding which encourage the interaction between the colorant and the media. Hydrogen bonding between color and media increases the strength of the color binding on the media. Furthermore, the hydroxyl groups of the cellulose may interact with the xcex4 cloud of an aromatic group on the colorant by hydrogen bonding.
Van der Waals forces are very weak when the interacting groups are far apart and a weak repulsion typically exists between the media and anionic dyes. The interaction between colorant and media becomes strong as the dyes start penetrating into the base sheet.
Electrostatic forces occur due to coloumbic attraction. The cationic groups on the media, such as Ti3+, Al3+, and Ca2+attract anionic dyes, such as water-soluble groups of SO32xe2x88x92, COO , and PO43xe2x88x92. The result is strong attraction between these groups, which causes an effective immobilization of the dye molecules, resulting in excellent print quality.
The xcex4xe2x80x94xcex4 interactions are very strong interactions that typically occur between dye molecules. These interactions normally generate either dye aggregation or crystallization6. If dye-dye interactions on the paper substrate are stronger than dye-paper interactions, dye may aggregate on the substrate causing printing problems. Thus a strong interaction between colorant and media is required.
6 Crystallization is a condition in which a dried ink film repels a second ink which must be printed on top of it. 
Hydrogen bonding and Van der Waals forces are the main interactions that occur in plain papers. Plain papers mainly consist of cellulose and therefore the main interactions are between the color and the cellulose. The penetration of color into the substrate will be controlled by capillary adsorption. If the paper has been internally or surface sized the rate of penetration of the colorant will be decreased which may lead to some ink bleeding and feathering problems.
The interaction of the colorant with coated paper is different however. The selection of the coating and ink formulation will have a significant effect on the ink absorption rate, image quality, and water/light fastness properties of the liquid ink. Electrostatic or ionic interactions play the key role in colorant coated paper interactions. Electrostatic interaction is stronger than hydrogen bonding and Van der Waals interactions. These interactions are more efficient, as the colorant is fixed in the vicinity where it was printed. The nature of the anionic dyes and the oxides will determine the print quality of ink jet printing since electrostatic interactions of the colorant with coated media occur between the anionic groups of the dyes and oxides. The binding energies of the dyes are greatly increased by electrostatic interactions resulting in high bonding strength.
Existing coated ink jet papers are mainly dependent on amorphous and gelled silica, which possess high micro porosity and macro porosity. The porous coating structure provides the driving force for the rapid diffusion of ink liquid into the coating layer and internal pore volume of the coating for storing large amounts of ink. These two properties interact to set the anionic dye at or near the coating surface, generating higher optical printing densities. The high surface area of the silica requires a strong binder to maintain adhesion to the paper and cohesion within the coating structure. Therefore, polyvinyl alcohol, the strongest binder available, is used.
Unfortunately, the current use of silica and polyvinyl alcohol has several limitations that effect the coating. The internal porosity of the silica pigments and the degree of hydrolysis of the polyvinyl alcohol limits running the coating solids at 20%. Silica pigments pose production problems and high cost because they must be coated at relatively slow speeds. Coating solids level is a major limiting factor with silica pigments because of viscosity, water absorption, and drying issues. Silica slurries alone do not usually flow well at levels above 15 to 20% solids, so dispersants are used to increase their concentrations. Also, silica has a great affinity for water given its high pore volume so it forms a paste as water is added until all the voids are filled. Only then is it fluid enough for the coating formulation. This behavior decreases the vehicle available for the slurry, so formulators must start at a lower solids concentration. The absorbed water in the pores also demands extra energy during drying. Calcium carbonate is another material sparingly used for ink jet printer coatings that dry similar to silica, but its surface area and void volume are much lower than silicaxe2x80x94resulting in inferior image quality. It is also abrasive and can exhibit poor coater runnability. Its use is limited to cast coated ink jet papers for glossy photo prints where it is used as a supplementary pigment to silica.
With the compositions for coating paper currently on the market higher quality coated ink jet papers must be coated off-machine and are not cost effective. Producing a paper sheet with the desired properties is difficult due to the need to find ways to coat ink jet paper on-machine at commercial speeds with no loss in quality. The preferred finished ink jet paper should be smooth, strong, opaque, bright, and able to handle the demands of ink jet printing while providing excellent print results, such as excellent ink adherence, high scratch and ink resistance, and bleed control for sharp edges. It was therefore necessary to develop the composition for coating paper of the present invention that produces a coated paper that overcomes the disadvantages of the existing art while presenting a high print quality image at a reduced cost. More specifically, the present invention contemplates substituting a zeolite pigment for silica in matte ink jet coating formulations.
A zeolite pigment that possesses the desirable combination of brightness, color, particle size distribution, surface area, internal void volume, rheology and hardness could also be useful in overcoming the limitations of conventional and other specialty pigments in various papermaking and paper coating applications including but not limited to: (1) toner bond improvement in laser and other dry toner imaged digital papers; (2) elimination of smudging and improvement of print quality in direct print flexography on coated linerboard used in corrugated containers; (3) elimination of print through on newsprint and ultra light weight coated papers; (4) improvement of dot fidelity and print quality on coated rotogravure printing papers; (5) low abrasion extender for titanium dioxide pigments; (6) improvement of coefficient of friction of paper and paperboard; (7) production of technical specialty papers such as anti-tarnish, gas filtration, and absorbent papers with improved properties and lower cost of manufacture; (8) more economical microparticulate retention system chemistry; (9) additive to improve the efficiency of deinking systems.
Zeolites are crystalline, hydrated aluminosilicates of the alkali and alkaline earth metals. More particularly, zeolites are framework silicates consisting of interlocking tetrahedrons of SiO4 and AlO4. In order to constitute a zeolite the ratio of silicon and aluminum to oxygen must be xc2xd. The alumino-silicates structure is negatively charged and attracts the positive cations that reside within. When exposed to higher charged ions of a new element, zeolites will exchange the lower charged element contained within the zeolite for a higher charged element. Unlike most other tectosilicates, zeolites have large vacant spaces or cages in their structures that allow space for large cations such as sodium, potassium, barium, and calcium and relatively large molecules and cationic molecules, such as water, ammonia, carbonate ions, and nitrate ions. In most useful zeolites, the spaces are interconnected and form long wide channels of varying sizes depending on the mineral. These channels allow ease of movement of the resident ions and molecules into and out of the structure.
Zeolites are characterized by 1) a high degree of hydration, 2) low density and large void volume when dehydrated, 3) stability of the crystal structure of many zeolites when dehydrated, 4) uniform molecular sized channels in the dehydrated crystals, 5) ability to absorb gases and vapors, 6) catalytic properties, and 7) cation exchange properties.
The use of natural zeolites in paper making has a long history, but has been almost unique to Japan where zeolite has been used as filler to improve bulkiness and printability. Natural zeolites have also been used as fillers for paper in Hungary. These natural zeolites however are a low brightness material and this renders it unsatisfactory for application in the United States on coated ink jet paper where high brightness is expected.
Numerous families of natural zeolites exist and each has varying characteristics. Unfortunately, natural zeolites exhibit nonuniform properties that makes them difficult to work with in many applications because ores from one location can vary with any other. It is however possible to manufacture zeolites with uniform properties. The preferred zeolite for use in the present invention is a processed form of the natural mineral clinoptilolite which is a hydrated sodium potassium calcium aluminum silicate having the formula (Na, K, Ca)2-3 Al3 (Al,Si)2 Si13)36-12H2O. This zeolite is within the family Heulandite that also includes the mineral heulandite which is a hydrated sodium calcium aluminum silicate. The physical characteristics of raw clinoptilolite are listed in Table 1.
Clinoptilolite""s structure is sheet like with a tectosilicate structure where every oxygen is connected to either a silicon or an aluminum ion (at a ratio of [Al+Si]/0=xc2xd). The sheets are connected to each other by a few bonds that are relatively widely separated from each other. The sheets contain open rings of alternating eight and ten sides. These rings stack together from sheet to sheet to form channels throughout the crystal structure. The size of these channels controls the size of the molecules or ions that can pass through them. Clinoptilolite is well suited for various applications, such as in paper coating compositions, because it exhibits large pore space, high resistance to extreme temperatures, and has a chemically neutral structure.
An object of the present invention is to provide a novel purified natural zeolite pigment for coated ink jet papers and digital printing papers to replace silica pigments.
Another object of the present invention is to provide a novel purified natural zeolite pigment that can be used as a specialty coating pigment in coated linerboard for direct post print flexography to prevent smudging and to improve image fidelity.
Still another object of the present invention is to provide a novel purified natural zeolite pigment that can act as a supplementary coating pigment in ultra lightweight coated publication papers.
It is yet another object of the present invention to provide a novel purified natural zeolite pigment that can act as a supplementary coating pigment for water based gravure7 printing papers.
7 Gravure printing is a method of printing using etched metal cylinders. 
An additional object of the present invention is to provide a novel purified natural zeolite pigment that can replace calcined kaolin as a titanium dioxide extender in coated recycled paperboard and coated solid unbleached sulfate (SUS) beverage carrier stock.
It is still another object of the present invention to provide a novel purified natural zeolite pigment that can act as filler in newsprint to prevent print-through.
It is yet another object of the present invention to provide a novel purified natural zeolite pigment that can act as filler in specialty technical papers such as anti-tarnish, gas filtration, filter, and absorbent papers.
Another object of the present invention is to provide a novel purified natural zeolite pigment that can be used as a microparticulate retention aid.
Still another object of the present invention is to provide a novel purified natural zeolite pigment that can be used as a deinking aid in combination flotation-washing systems.
Yet another object of the present invention is to provide a novel purified natural zeolite pigment that can be used as a coefficient of friction (COF) control aid in recycled linerboard.
Another object of the present invention is to provide a novel purified natural zeolite pigment for use in a coating composition that has improved rheology compared to silica and other specialty pigments.
Still another object of the present invention is to provide a novel purified natural zeolite pigment for use in a coating composition that improves coater runnability.
It is yet another object of the present invention to provide a novel purified natural zeolite pigment for use in a coating composition that has decreased energy consumption in drying.
It is an object of the present invention to provide a novel composition for coating paper that has water slurries with a higher percentage of solids and good shear thinning rheology compared to existing compositions.
Another object of the present invention is to provide a novel composition for coating paper that has higher coating formulation solids compared to existing compositions.
Still another object of the present invention is to provide a novel composition for coating paper that has enhanced on-machine coating run ability and therefore enhanced production rates over existing compositions.
It is yet another object of the present invention to provide a novel composition for coating paper that has low Einlehner abrasion which results in reduced wear to process equipment and no metallic marks are left on the paper by the gripper bars.
Another object of the present invention is to provide a novel composition for coating paper that has a low bulk density.
Still another object of the present invention is to provide a novel composition for coating paper that has faster on-machine drying rates because of higher percent solid coatings than existing compositions which results in lower drying costs and reduced print smear.
Yet another object of the present invention is to provide a novel composition for coating paper that has a low crystalline silica content.
It is another object of the present invention to provide a novel composition for coating paper that coats with essentially no dusting.
It is still another object of the present invention to provide a novel composition for coating paper that has improved first pass retention in paper machine trials compared to existing compositions.
Another object of the present invention is to provide a novel composition for coating paper that has improved optical/reflective densities of four-color cyan, magenta, yellow, black (CMYK) ink jet print.
An additional object of the present invention is to provide a novel composition for coating paper that makes lighter coat weights possible because of higher internal void volume.
Still another object of the present invention is to provide a novel composition for coating paper with a slightly basic pH.
Yet another object of the present invention is to provide a novel composition for coating paper that has a high brightness of 90% or more.
Another object of the present invention is to provide a novel composition for coating paper that has a narrow particle size distribution with few fines.
An additional object of the present invention is to provide a novel composition for coating paper that improves ink jet print density.
It is yet another object of the present invention to provide a novel composition for coating paper that improves ink receptivity in printing papers.
Still another object of the present invention is to provide a novel composition for coating paper that has improved opacity.
An additional object of the present invention is to provide a novel composition for coating paper that has less soak-in and reduced roughening of the base sheet during application which results in a smoother coated sheet.
Another object of the present invention is to provide a novel composition for coating paper that allows higher operating speeds and higher production rates.
It is still an additional object of the present invention to provide a novel composition for coating paper that has the capability to coat on high speed paper machines rather than only on low speed off machine coating lines which reduces waste and costs.
In satisfaction of these and related objectives, Applicant""s present invention provides a purified natural zeolite pigment composition for coating and/or filling of paper. Applicant""s invention permits its practitioner to manufacture coated paper for use in ink jet printers that exhibits improved characteristics over existing uncoated and coated papers such as high print quality images and reduced cost. It also permits the practitioner to make other specialty and technical papers that exhibit quality and economic advantages over papers made with existing technology and commercially available materials.